1. Field of the Invention
The invention relates to the catalytic copolymerization of ethylene with high activity Mg and Ti containing complex catalysts in a gas phase process to produce ethylene copolymers having a density of .gtoreq.0.91 to .ltoreq.0.94 and a melt flow ratio of .gtoreq.22 to .ltoreq.32.
2. Description of the Prior Art
Until recently, low density (.ltoreq.0.940) polyethylene has been produced commercially, for the most part, by the high pressure (.gtoreq.15,000 psi) homopolymerization of ethylene in the gas phase in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. On a world wide basis, the amount of low density polyethylene produced in this fashion, annually, amounts to more than thirteen (13) billion pounds.
As recently disclosed in U.S. Pat. No. 4,011,382 and in Belgian Pat. No. 839,380 it has been found that low density polyethylene can be produced commercially at pressures of &lt;1000 psi in a gas phase reaction in the absence of solvents by employing selected chromium and titanium (and, optionally, fluorine) containing catalysts under specific operating conditions in a fluid bed process.
The products produced by the processes of U.S. Pat. No. 4,011,382 and Belgian Pat. No. 839,380, however, have a relatively broad molecular weight distribution (Mw/Mn) of .gtoreq.6 to .ltoreq.20. As such, although readily useful for a large number of applications in the areas of wire and cable insulation and molded pipe they are not broadly useful in the areas of injection molding applications. They are also not broadly used in the area of film applications because of the poor optical and mechanical properties of films made from such resins.
To be commercially useful in a gas phase process, such as the fluid bed process of U.S. Pat. Nos. 3,709,853; 4,003,712 and 4,011,382 and Canadian Pat. No. 991,798 and Belgian Pat. No. 839,380, the catalyst employed must be a high activity catalyst, that is, it must have a level of productivity of .gtoreq.50,000, and preferably .gtoreq.100,000, pounds of polymer per pound of primary metal in the catalyst. This is so because such gas phase processes usually do not employ any catalyst residue removing procedures. Thus, the catalyst residue in the polymer must be so small that it can be left in the polymer without causing any undue problems in the hands of the resin fabricator and/or ultimate consumer. Where a high activity catalyst is successfully used in such fluid bed processes the heavy metal content of the resin is of the order of .ltoreq.20 parts per million (ppm) of primary metal at a productivity level of .gtoreq.50,000 and of the order of .ltoreq.10 ppm at a productivity level of .gtoreq.100,000 and of the order of .ltoreq.3 ppm at a productivity level of .gtoreq.300,000. Low catalyst residue contents are also important where the catalyst is made with chlorine containing materials such as the titanium, magnesium and/or aluminum chlorides used in some so-called Ziegler or Ziegler-Natta catalysts. High residual chlorine values in a molding resin will cause pitting and corrosion on the metal surfaces of the molding devices. Cl residues of the order of .gtoreq.200 ppm are not commercially useful.
U.S. Pat. No. 3,989,881 discloses the use of a high activity catalyst for the manufacture, under slurry polymerization conditions, of ethylene polymers having a relatively narrow molecular weight distribution (Mw/Mn) of about 2.7 to 3.1. Attempts were made to use catalysts similar to those described in U.S. Pat. No. 3,989,881 for the purpose of making polyethylene of narrow molecular weight distribution by polymerizing ethylene alone or with propylene in the gas phase in a fluid bed process using apparatus and conditions similar to those employed in U.S. Pat. No. 4,011,382 and Belgian Pat. No. 839,380. These attempts were not successful. In order to avoid the use of the solvents in the slurried catalyst systems of U.S. Pat. No. 3,989,881 the Ti/Mg containing components were dried. However, the dried material, a viscous, gummy, pyrophoric composition, could not be readily fed to the reactor because it was not in a free flowing form. Even when blended with silica, to improve its free flowing properties and then added to the reactor, the results were commercially unacceptable. The productivity of the catalyst was poor, or the catalyst was pyrophoric and difficult to handle, or the polymer product had a low bulk density i.e., of the order of .ltoreq.6 pounds/cubic foot.
Polymers of such low bulk density are not commercially desirable because they are fluffy. If the polymer is to be stored or sold in granular form, significantly larger amounts of storage and shipping space is required for handling these materials. Even if the granular polymer is to be pelletized prior to shipping, the processing of a given quantity of the low bulk density material through the pelletizing equipment requires significantly longer processing times than would the same quantity of high bulk density materials, when using the same extrusion equipment.
U.S. Pat. No. 4,124,532 discloses the polymerization of ethylene and propylene with high activity catalysts. These catalysts comprise complexes which may contain magnesium and titanium. These complexes are prepared by reacting the halide MX.sub.2 (where M may be Mg) with a compound M'Y (where M' may be Ti and Y is halogen or an organic radical) in an electron donor compound. These complexes are then isolated by either crystallization, by evaporation of the solvent or by precipitation.
Polymerization is carried out with these catalytic complexes and an alkyl aluminum compound.
However, U.S. Pat. No. 4,124,532 does not disclose any special techniques or methods of preparing the catalyst in order to achieve the desirable results described in the present invention. The use of the catalysts described in U.S. Pat. No. 4,124,532, without these special methods, would not lead to a commercial fluid bed process to produce polyethylenes at commercial rates. In addition the examples in the gas phase do not describe a practical process of copolymerization to produce the special low density copolymers with attractive polymer morphology described in the present invention.
U.S. Pat. Nos. 3,922,322 and 4,035,560 disclose the use of several Ti and Mg containing catalysts for the manufacture of granular ethylene polymers in a gas phase fluid bed process under a pressure of &lt;1000 psi. The use of these catalysts in these processes, however, has significant disadvantages. The catalyst of U.S. Pat. No. 3,922,322 provides polymers having a very high catalyst residue content i.e., about 100 ppm of Ti and greater than about 300 ppm Cl, according to the working example of this patent. Further, as disclosed in the working example of U.S. Pat. No. 3,922,322, the catalyst is used in the form of a prepolymer, and very high volumes of the catalyst composition must be fed to the reactor. The preparation and use of this catalyst thus requires the use of relatively large sized equipment for the manufacture, storage and transporting of the catalyst.
The catalysts of U.S. Pat. No. 4,035,560 also apparently provide polymers having high catalyst residues, and the catalyst compositions are apparently pyrophoric because of the types and amounts of reducing agents employed in such catalysts.
U.S. patent application Ser. No. 892,325 filed Mar. 31, 1978, now abandoned, and refiled as Ser. No. 014,414 on Feb. 27, 1979, in the names of F. J. Karol et al and entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor, and which corresponds to European Patent Application No. 79100953.3 which was published as Publication No. 004,645 on Oct. 17, 1979, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are polymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced have excellent physical properties which allow them to be used in a broad range of molding applications. However these polymers have several disadvantages. First, because of the presence of the support material in the catalyst which is not removed from the polymer prior to the molding thereof, the polymer containing certain of these support materials is not too useful for clear film applications. These support particles may impart poor film rating values to clear films made from such polymers. Second, the polymers, particularly at the lower polymer densities, also have a relatively low bulk density. The handling of these polymers therefore requires the use of larger volumes of shipping and storing equipment than is required for the pelleted products which the molding industry is more accustomed to handling. As a result larger capital investments are needed for the equipment needed to handle and store these low bulk density granular materials. Further, the feeding of the low bulk density granular materials to molding and extrusion equipment requires longer feed times than is required for the same weight of pelleted material because of the larger volumes of the granular material that are involved. Third, the polymer particles formed during the fluid bed polymerization process are irregular in shape and are somewhat difficult to fluidize. The final product also contains a relatively high level of fines, i.e., particles having a particle size of .ltoreq.150 microns.